1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly, to a paper feeding unit to supply paper to a printing unit in a main body, and an image forming apparatus having the same.
2. Description of the Related Art
Generally, an image forming apparatus, such as a laser printer, an ink-jet printer, a multi-function printer, a copying machine, or the like, includes a main body that forms an external appearance, a printing unit mounted in the main body to form an image by applying a toner or an ink to paper, a paper feeding unit to supply the paper to the printing unit, and a paper discharge unit to discharge the printed paper to the outside of the main body.
The paper feeding unit includes a paper feeding cassette or a paper feeding tray, on which the paper is loaded, and a pickup device to pickup the paper loaded on the paper loading device sheet by sheet and to feed the paper toward the printing unit. The pickup device is provided with one or more pickup rollers made of a rubber material having a large frictional coefficient. The paper feeding cassette is loaded with paper having the same size for automatic paper feeding, and is ideal for relatively large, routine print jobs. The paper feeding tray is also loaded with paper, but allows a user to swiftly and manually load one or more sheets of paper of varying sizes as needed, which makes the paper feeding tray ideal for relatively small, non-routine print jobs.
Typically, the paper feeding cassette is removably mounted in a mounting space provided in a lower portion of the main body of the image forming apparatus. A knock-up plate, on which the paper is loaded, is mounted in the paper feeding cassette. One end portion of the knock-up plate is supported by a spring, and the other end portion is hingedly coupled to a portion of the paper feeding cassette. By rotating on the hinge portion at a predetermined angle, the knock-up plate pushes up one end of the paper loaded thereon toward the pickup device.
However, in the conventional paper feeding cassette, because the knock-up plate is elastically supported by the spring with a constant elastic force all of the time, a paper feeding force of the pickup device to pick up the paper and to feed the paper to the printing unit is constant. The paper feeding force can be arithmetically derived from a frictional coefficient and a normal force of the pickup roller. The normal force is equivalent to a value of subtracting a total weight of the knock-up plate and the sheets of paper on the knock-up plate from the elastic force of the spring.
Accordingly, assuming that the elastic force of the spring and the weight of the knock-up plate are scarcely changed, the weight of the sheets of paper on the knock-up plate has a considerable influence on the paper feeding force. Such a phenomenon depends largely on a size of the paper. For example, in case of the most commonly used A4, Letter/Legal, and B5 paper, since a difference in size therebetween is very small, a large change of the paper feeding force does not occur. However, when the paper having a relatively small size, e.g., A5 paper, is used, because the weight of the sheets of paper on the knock-up plate decreases greatly, a problem occurs such that the paper feeding force considerably increases. The increase in the paper feeding force causes multiple feeding of the paper.